Spent catalyst distributors are known for many years. In the Fluidized Catalytic Cracking (FCC) process, the catalyst comes in contact with atomized heavy hydrocarbon molecules in the riser. In the riser, at high temperature, cracking occurs and coke gets deposited on the catalyst. Thereafter, the catalyst enters into stripper for removing the entrapped hydrocarbons inside the catalyst pores. The coke laden catalyst leaving the stripper is hereby being referred as “spent catalyst”.
The coke on the spent catalyst, which covers the active sites on the catalyst and thereby substantially reduces the activity of the catalyst. It is possible, however, to remove the coke from the catalyst inside the regenerator by burning with air. The spent catalyst is fed into the regenerator through a “Spent Catalyst Distributor” (SCD) for uniform burning of the catalyst inside the Regenerator. In the Regenerator, at high temperature and in the presence of excess air, the coke deposited on the catalyst gets converted into CO, CO2. The regenerated catalyst with significantly lesser coke content is again being circulated to riser for cracking.
It is a known problem in the prior art that if the spent catalyst is not distributed into the regenerator in a uniform manner, the catalyst is not regenerated as efficiently, after-burning and the associated temperature increase in the dilute phase of the regenerator occurs, and NOx, SOx are generated in undesirable amounts. Several past patents have attempted to resolve the deficiencies of the prior art distribution problems with varying degrees of success.
U.S. Pat. No. 5,773,378 discloses a device to distribute spent catalyst at the lower end of a spent catalyst standpipe. The standpipe enters the regenerator from the side wall, near the top of the bed level, conveying the catalyst through a horizontal conduit to the center of the regenerator, followed by a vertically downward conduit with a deflector plate end cap, and discharging catalyst through a plurality of discrete radial slots on the lower side wall of the vertical conduit.
U.S. Pat. No. 5,635,140 covers a self-aerating spent catalyst distributor to discharge catalyst radially and downwardly from a center-well via lipped trough arms into the catalyst bed. U.S. Pat. No. 6,503,460 describes a FCC regenerator countercurrent equipped with an internal baffle separating the dense phase of the fluidized bed in two parts. While the primary objective of the baffle is to reduce back mixing between the upper and lower sections of the fluidized bed. The baffles also facilitate the reduction in the diameter of the rising bubbles of gas passing through the dense bed, just before bursting at the surface of dense catalyst bed. This leads to reduction in the entrainment of catalyst.
U.S. Pat. No. 6,809,054 discloses a design of SCD distributor arrangement for introducing spent fluid catalytic cracking catalyst more uniformly across the dense bed of a regenerator to provide more even contact with regeneration gas in order to avoid hot spots and zones of incomplete combustion. In this design, a spent catalyst carrying arm extends out from the wall of the regenerator and extends towards the opposite wall. This invention also considers the use of aeration flow to fluidize the extended arm to assist catalyst flow. This patent also teaches that small arms can extend from the main arm at angles of 90° or 60°. However, this design, while not in the shape of a spooked wheel, also causes the catalyst to be distributed in the regenerator in a very uneven manner.
U.S. Pat. No. 6,797,239 covers a spent catalyst distributor design where spent catalyst and transport gases moves upwards via spent catalyst riser and diverted in a radial outward direction via a deflector cone. Such design of SCD cannot provide even distribution of spent catalyst across the regenerator shell.
U.S. Pat. No. 8,349,753 describes a bath tub type spent catalyst distributor which is half open and contains troughs and sub-troughs branched off the main trough for enhanced catalyst distribution. Down flow tubes are extended from the main trough and the sub trough for flow of catalyst. Such design of SCD works when the SCD is half filled. The catalyst overflows only from the top of SCD as the far end of the SCD is blocked.
The designs which are disclosed in the above prior art limits the distribution of the catalyst at a single location or the distribution limited to narrow portion of the regenerator.